1. Field of the Invention
This invention relates to electrochemical cells such as fuel cells or electrolytic cells, such as chlor-alkali electrolytic cells utilizing air or oxygen depolarized composite cathodes.
2. Description of the Prior Art
Electrodes for use in electrochemical cells, particularly air or oxygen diffusion cathodes for use in chlor-alkali electrolytic cells which are based upon carbon in combination with wetproofing agents such as polytetrafluoroethylene, are subject to failure as the result of the excessive wetting of the electrochemically active hydrophilic portion of the electrode by the electrolyte. This excessive wetting is often partially counteracted by laminating the electrochemically active layer to a porous, hydrophobic (backing) layer. The pores of the hydrophobic layer pass air or oxygen so as to make the air or oxygen available to the electrochemically active layer of the electrode.
The air or oxygen gas diffusion cathode in a chlor-alkali electrolysis cell is more energy efficient than electrodes which do not pass air or oxygen to effect depolarization since in gas diffusion electrodes the unwanted by-product (hydrogen) in the electrolysis of an alkali metal halide is eliminated. The air or oxygen diffusion cathodes not only eliminate the production of hydrogen at the cathode but force the formation of desirable hydroxide ions.
It is known to form electrochemically active layers in an electrode utilizing as an active component an electrically conductive carbon black. The tendency of the electrically conductive carbon black to be wetted by the electrolyte must be controlled within limits in order to provide an electrode with a reasonable period of usefulness. The efficiency of the carbon black active layer is reduced during operation of the cell as the pores of the carbon black become completely wet out by the electrolyte since the air or oxygen necessary for depolarization of the active layer of the electrode does not under complete wetting conditions penetrate sufficiently into the active portion of the electrode so as to effect depolarization.
Prior art carbon-based hydrophilic electrolyte-active layers of air or oxygen depolarized electrodes have been rendered less susceptible to wetting by the electrolyte by blending a hydrophobic polymer such as polytetrafluoroethylene with carbon to impart hydrophobicity to the carbon and thus increase the useful life of the electrode. It is thus common to employ mixtures of carbon black and polytetrafluoroethylene in particulate form in the preparation of the electrolyte active layer of the electrode. Additionally, a homogeneous, porous, hydrophobic layer of polytetrafluoroethylene has been employed in prior art gas diffusion electrodes on the side of the electrode facing away from the electrolyte. In a chlor-alkali electrolytic cell, this side of the gas diffusion electrode is normally in contact with air or oxygen under pressure. The pressure helps to maintain gas penetration into the electrolyte-permeable layer, particularly in areas of the electrode having large pores.
Heretofore, these methods of increasing the useful life of gas diffusion electrodes have not been entirely satisfactory and nearly complete wetting of the electrolyte-permeable layer of the electrode ultimately takes place over a relatively short period of time causing failure of the electrode.
In U.S. Pat. No. 4,058,482 and U.S. Pat. No. 4,150,076, a fuel cell electrode is disclosed in which a sheet material is formed from co-agglomerates of a polymer such as polytetrafluoroethylene and a pore-former. This is subsequently bonded to a support plate to define the fuel cell electrode. The disclosed co-agglomerates replace the earlier polytetrafluoroethylene suspensions previously employed in the preparation of fuel cell electrodes. In the preparation of the electrode, the co-agglomerates are mixed with a catalyst suspension and surfactant-coated co-agglomerates and subsequently the mixture is filtered and the filter cake bonded to support plate which has been treated with fluorinated ethylene propylene copolymer to impart the requisite structural integrity to the resulting fuel cell electrode. The electrode is dried and sintered and thereafter the pore-former is leached out by immersing the electrode in an acid which will attack the pore-forming ingredient. Zinc oxide is disclosed as a useful pore-forming ingredient. The support plate referred to above is defined as a sheet material consisting of carbon fibers which have been bonded together to form a gas-permeable paper-like structure having sufficient strength to function as a support for the electrode.